Transparent conductive oxides (TCO) are metal (or mixtures of metals) oxides that possess the usually mutually exclusive properties of high transparency and electrical conductivity. TCO materials are transparent to electromagnetic radiation in the visible region of the spectrum due to a high optical bandgap. At the same time, the electrical conductivity is good due to high electron mobility. TCO materials include, for example, tin-doped indium oxide (ITO), aluminum-doped zinc oxide (AZO), boron-doped zinc oxide (BZO) and fluorine-doped tin oxide (FTO), by way of example and not limitation.
Intrinsic to optoelectronic devices is the interaction of light with an electrically active component. Such devices include photovoltaic (PV) cells, photodiodes, flat panel displays, touch screens, light emitting diodes, phototransistors, semiconductor lasers, and the like. Typically, such devices must contain at least one electrically conductive electrode that is transparent to light. A TCO coating over a transparent, non-conductive, glass substrate may provide an essential coating material for such applications. TCO coatings on transparent substrates may also be used for transparent heating elements, antistatic coatings, or electromagnetic shielding.
Currently, the majority of photovoltaic solar cells manufactured are based on a substrate of crystalline silicon, with layers of p-doped and n-doped silicon forming a p-n junction such that absorption of ultraviolet, visible and infrared light results in a voltage across the cell. At least one side of a cell must be transparent to light in order to function, and typically a thin coating of a non-conductive oxide or nitride forms the outermost layer of a cell. Properly designed, this layer both passivates defects on the surface of the silicon and reduces reflection of light that would cause loss of power generation. TCO coatings may be used as the front and/or back sides of photovoltaic solar cells. A TCO coating offers the advantage that the entire front and/or back surfaces of the cell are electrically conductive, allowing for efficient collection of electric current, while still functioning as an anti-reflective coating. One such solar photovoltaic cell that uses such a TCO coating is what is known as a silicon heterojunction (SHJ) cell, in which the base substrate of the cell comprises a crystalline silicon wafer, with an amorphous, intrinsic (i-type) silicon thin film layer deposited on the crystalline silicon, and doped amorphous layers of silicon deposited over the intrinsic layer providing a p-n junction. This cell technology is described, for example, in U.S. Pat. No. 8,283,557 to Yu et al., U.S. Pat. No. 7,960,644 to Sinha, U.S. Pat. Pub. No. 2012/0305060 to Fu et al., and U.S. Pat. Pub. No. 2012/0097244 to Adachi et al., the subject matter of each of which is herein incorporated by reference in its entirety.
In order for electrical current to be collected for power generation, electrical contact to both sides of the cell to an external circuit must be made. The contacts typically comprise a metallic pattern in ohmic contact with the device.
The ideal contacting pattern will have:
(1) high conductivity in order to minimize resistive losses;
(2) low contact resistance with the substrate;
(3) low surface area in order to minimize shading losses; and
(4) high adhesion to the substrate to ensure mechanical stability.
In order to obtain maximum efficiency, the entire surface of a photovoltaic cell would ideally be covered by highly conductive material. However, pure metals possess very high reflectivity and absorption of light, rendering them unsuitable as blanket coatings. While TCO coatings offer both transparency and electrical conductivity, the bulk resistivity of TCO (approximately 100 μΩ-cm for ITO) is still much greater than pure metals, leading to high resistive losses and efficiency loss due to the sheet resistance of a thin TCO film. In addition, these losses become more severe as the area size of the device becomes larger.
In order to reduce resistive losses and improve current collection, a metallic grid comprising fingers and busbars may be fixed in contact with the TCO such that ohmic contact is made between the grid and the TCO. This grid results in partial shading of light from the device, resulting in loss of power. Thus, the area of the grid is generally kept to a minimum.
Silver paste is a common conductor material for collection of electrical current from the cell. The paste can be screen printed in the desired grid pattern of fingers and busbars, dried, and sintered at high temperature. Although this offers the advantages of high throughput and low contact resistance, it also suffers the disadvantage of higher bulk resistivity as compared with pure metals. Glass frit material can be added to improve mechanical properties (including adhesion), but this results in decreased conductivity. A dense, solid metallic conductor grid material would therefore be advantageous. However, attachment of metals to TCO coatings is problematic, because metals typically form contacts to TCOs that exhibit very low adhesion.
U.S. Pat. No. 4,586,988 to Nath et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method and compositions for the deposition of nickel, copper, and other metals onto ITO substrates. However, non-adherent layers are obtained when these metals are plated onto ITO substrates.
U.S. Pat. No. 4,824,693 to Schlipf et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method of depositing a metallic conductor on ITO on glass substrates by electroless metallization. The ITO is activated by treatment with a solution of colloidal palladium followed by electroless plating of nickel. However, such a method suffers from several disadvantages. Treatment with colloidal palladium tends to non-selectively activate both conductive and non-conductive substrates, causing unwanted metal deposition to occur in some areas. In addition, metal layers deposited by electroless plating generally have poor adhesion.
U.S. Pat. No. 5,384,154 to De Bakker et al., the subject matter of which is herein incorporated by reference in its entirety, also describes a method for deposition of metals onto ITO on glass substrates, where the ITO is activated by treatment with a colloidal palladium solution followed by electroless nickel plating. This method also produces metal layers generally having poor adhesion.
U.S. Pat. Pub. No. 2010/0065101 and U.S. Pat. Pub. No. 2012/0181573 both to Zaban et al., the subject matter of each of which is herein incorporated by reference in its entirety, describe methods for electroplating metals onto TCO coatings, wherein the metal electroplating is preceded by an “electrolysis reduction” step in which cathodic current is supplied to the substrate in the absence of platable metal ions, causing partial reduction of metal cations in the TCO to metal, and subsequently electroplating nickel, cobalt or copper, which was reported to improve adhesion. However, it was found that such a step may easily damage the TCO, causing degradation of electrical and mechanical properties.
Lukyanov et al., Proc. 27th European Photovoltaic Solar Energy Conference and Exhibition, 1680 (2012), reported that direct electroplating of copper onto ITO-coated photovoltaic cells resulted in poor adhesion if the copper layer had a thickness of greater than 500 nm.
Thus, there remains a need in the art for an improved method of electroplating metals onto TCO substrates that overcomes the deficiencies of the prior art.